1. Field of the Invention
The present invention relates to a printing device and a printing method.
2. Description of the Related Art
Conventionally, so-called flatbed-type printing devices are known. In a flatbed-type printing device, a printing head is moved, for example, in two directions perpendicular to each other in a plane with respect to a printing subject placed on a table. Such a flatbed-type printing device is used for performing printing on, for example, a printing subject such as a substantially rectangular business card, greeting card or the like. In the following description, the term “printing subject” is a “substantially rectangular sheet-type or plate-type printing subject such as a substantially rectangular business card, greeting card or the like”, unless otherwise specified.
For performing printing on a printing subject by use of a flatbed-type printing device, the printing subject is placed on a table and then printing is performed. For accurate printing, the printing subject needs to be placed precisely at a predetermined position. This requires, for example, measuring the size of the printing subject beforehand, so that the position at which the printing subject is to be placed is determined accurately.
Such a work needs to be performed accurately. For an unexperienced operator, the work is time-consuming. This causes a problem that the printing requires a long time and the production cost is raised. There is also a problem that the work requires a great number of steps to be performed by an operator, which imposes a heavy load on the operator.
A technology for solving these problems is proposed by, for example, Japanese Laid-Open Patent Publication No. 2007-136764. According to the technology disclosed in Japanese Laid-Open Patent Publication No. 2007-136764, a jig that can be secured to a table and accommodate a plurality of printing subjects at a fixed position is produced. For performing printing, the jig is secured to the table and a plurality of printing subjects are accommodated in the jig. The position in the jig at which each of the plurality of printing subjects is accommodated is predetermined. The position is input beforehand to a microcomputer that controls the printing device. This allows the position of each printing subject to be determined by the jig, so that printing is performed at predetermined positions of the printing subjects.
However, the above-described technology requires producing a jig in accordance with the shape or the size of a printing subject. This causes a problem that the production of a jig is time-consuming. In addition, even in the case where printing is to be performed on a small number of printing subjects, a jig needs to be produced. This causes a problem that in the case where printing is performed on a small number of printing subjects, the cost per printing subject is increased.
A conceivable measure for solving these problems is to acquire the position and the posture of the printing subject located on the table and determine the position at which the printing is performed based on the acquired information on the position and the posture. With this measure, the position and the posture of the printing subject may be acquired by extracting a difference between an image obtained when no printing subject is placed on the table and an image obtained when the printing subject is placed on the table. In other words, a so-called background subtraction method is usable. However, in the case where the table and the printing subject have similar colors or in the case where there is a shadow between the table and the printing subject, the background subtraction method does not provide the shape or the like of the printing subject accurately.
A technology for acquiring an accurate shape of an object (corresponding to the printing subject) by use of the background subtraction method is disclosed in, for example, Japanese Patent No. 4012200. According to this technology, a background image (corresponding to the image obtained when no printing subject is placed on the table) is captured at a plurality of time points, and the shape or the like of the object is acquired by use of the background images captured at the plurality of time points. However, this technology requires a great number of images having different levels of luminance since the luminance changes along with time. Use of such a great number of images requires a long time for image capturing and also a large memory capacity. A process of acquiring the position or the posture of the printing subject is also time-consuming. In addition, almost no ambient light is incident into the inside of the printing device. In such an environment where the luminance does not change almost at all, it is considered difficult to stably acquire the shape or the like of an object.